Bright Annealing of 430F(R) 3

[dgallup]

We have several components made from 430F & 430FR that require a bright anneal for both magnetic & assembly reasons. We have a local heat treater who does a great job for us but will not tell us his exact process. We subcontract the manufacture of some of these parts and the subcontractors are having trouble finding local heat treaters that can produce a consistently bright finish.
Here are some excerts from our bright annealing spec but it does not seem to be sufficient to get consistently bright parts. Times & temperatures are from Carpenter. I suspect the atmosphere is not adaquate. Any suggestions are welcome!

3.3.2 Annealing
Temperature, time, atmosphere and rate of cooling shall be as specified in Sections 3.3.2.1 through 3.3.2.3.

3.3.2.1 Annealing Temperature and Time
The parts shall be heated to the temperature range in
the chart below and held at the selected temperature
for 2 to 4 hours.

Silicon content Annealing Temperature
Less than 1% 788-843°C (1450-1550°F)
1% or more 843-1000°C (1550-1830°F)

Note: Furnace control thermocouple(s) may be used
for the temperature and time measurements provided
sufficient allowance is made to ensure all the parts in
the load reach the selected temperature for the
specified time.

3.3.2.2 Atmosphere
A dry hydrogen or vacuum atmosphere shall be used
during the heating and soaking periods of the cycle.
If a vacuum atmosphere is used, of 1X10-3 millimeters
(1 micron) of mercury or less shall be maintained
throughout the soaking period.

3.3.2.3 Cooling Rate
Furnace cool to 425°C (800°F) at a rate not
exceeding 56°C (100°F) per hour. Below 425°C
(800°F) cooling rate is optional. Nitrogen with a dew
point of -50°C (-60°F) or lower can be used to
assist cooling. Below 180°C (356°F) air cooling is
permissible.

3.5 Properties After Treatment
The characteristics and properties of the heat treated parts shall be in accordance with the requirements specified in Table 1, when tested in accordance with the applicable methods specified in Table 2. Unless otherwise specified herein, these properties apply to all portions of the part.

TABLE 1
REQUIREMENTS FOR HEAT TREATED PARTS
Hardness: 170 Max HV/10 kgf or 85 Max HRB

Surface Condition: Parts must be bright and clean. Discoloration in the form of an adherent oxide is not acceptable.

Dimensional Change and Distortion: Shall not exceed the limits necessary to meet finished part requirements.

 

[mcguire]

When you say the parts don't get a consistent bright finish, do you refer to oxides or reflectivity? Bright annealing doesn't produce the brightness;it only preserves the brightness which exists before the anneal. If you put a mirror finish in, you can expect a mirror finish out, but a matte finish going in will still be matte coming out.
The atmosphere should have a dew point no higher than -40C or 0.1 torr partial pressure of water. The cool down should be rapid because the atmosphere is less reducing at lower temperatures.

 

[dgallup]

I am refering to oxides, which are specifically disallowed in the requirements section. The problem with the oxides is that they are like fine grit sandpaper. These parts have press fits and/or o-rings have to slide over them. The oxides interfere with both.

The cool down rate is limited because we need near optimum magnetic properties. The .1 torr H2O partial pressure would be for the atmosphere in the furnace prior to pump down (760 torr absolute pressure), is that correct? As we are specifying a vacuum of .001 torr, it would not make sense to have .1 torr H2O partial pressure under vacuum.

 

[mcguire]

I need to get out the thermo book but I think the effectness of bright anneal vs vacuum is that the oxide is actually reduced by the atmosphere in a bright anneal, while in "vacuum" even trace amounts of oxygen or water vapor form stable chrome oxides.
Bright annealing isn't real hard to do well. I have not heard the same said of vacuum annealing.

 

[dgallup]

So how do you specify a bright anneal? I already have the word BRIGHT in the spec title, in three other parts of the spec & specifically disallow oxides but we still get parts with oxides on them!

 

[mcguire]

I would omit the vacuum alternative. You could put in a guidance for -40c max dewpoint also. See

http://www.rathmfg.com/Annealing.pdf

for a helpful diagram.

 

[dgallup]

Thanks, I'm coming to the same conclusion that bright annealing & vacuum annealing are mutually exclusive. I found the your link just a few minutes before your post, it looks like the best info to date. It looks like adding the dew point is necessary even with the dry hydrogen anneal.

I put the vacuum anneal option in the spec initially because I didn't want to restrict the heat treater unnecessarily. IF the part comes out bright & has the magnetic properties, I don't really care how it is accomplished. It looks like a lot of heat treaters only read the time/temperature part of the spec.

 

[dgallup]

After studying the chart a bit, it looks like the silicon in the 430F & 430FR may be contributing to the oxide problem. The intersection of Cr2O3 & 1000 deg C occur at a dew point of -37 deg C but the intersection of SiO2 & 1000 deg C is at a dew point of -78 deg C! Do I need to be concerned about silicon? 430FR is 1.0 to 1.5% silicon.

Is there a good way to remove the oxides? Citric passivation and electropolishing do not seem to be effective and I am concerned about size change, particularly on the press fit diameters (+/-.0004").

 

[mcguire]

check thread 330-122324 in the metallurgy forum on annealing 17-4. Nitric is the usual means for removing light scale.
Is this scale all the way to black? I'm concerned that you say it has significnt thickness. This just doesn't seem right. I wouldn't expect more than a light straw-colored tint from casual oxide formation.

 

[israelkk]

mcguire

The thickness problem is that when the plater is using chemical etching to remove the scale before passivation the process is not exact and close control is not possible. I had once very accurate parts to 0.01 mm tolerance aged in air furnace. When the scale was removes the parts were within -0.05 mm. Then another batch was aged in vaccum furnace with very light almost un-noticeable colloring result. Chemical etching to remove this scale resulted in the same -0.05 mm instead in less than 0.01.

 

[unclesyd]

To remove the oxides you might want to try the following procedure.

Immerse the part in a solution of 10% NaOH(W) + 3% KMnO4(W) at 80°C to 95°C for 30 minutes. Rinse well. The solution will turn green after a short time and become less effective.

A quick (10 seconds) immersion in 20% HNO3(V)+ 2% HF at room temperature. Rinse quickly with running water and the oxide should be loose but may have to scrubbed off. You may have to adjust the composition of the pickling solution and time of the immersion. Watch the time, keep it as short as possible. Dip twice in the pickling solution if needed .

I've used this procedure in cleaning 430 extrusion nozzles with over 1000, 0.0120" dia holes and still maintained the edge of the hole with no discernible radius at 100X .

Do this procedure in a lab with a shower and fume hoods if possible.
The appropriate PPG is required.
There is still a little art in cleaning scale as each condition is different
.
We purposely forced an oxide on our 430 SS components for enhanced corrosion resistance.

 

[mcguire]

These last two excellent posts show why the best way to deal with oxide is to not get it to begin with. A better bright anneal is in order.

 

[metman]

"Thanks, I'm coming to the same conclusion that bright annealing & vacuum annealing are mutually exclusive."

Yes this is true to a "degree." Brightness is relative unless you specify the degree. To some customers a vacuum anneal would qualify as bright anneal but not in your case. You definitley need H2 atmosphere with low dewpoint as suggested for reducing oxides.

Also to corroborate the comment about prefinish condition before annealing. You need to alert your subcontractors to this fact and that they make sure the parts are properly degreased prior to annealing.

 

[EdStainless]

The generic deffinition of Bright Anneal just requires a protective atmosphere to maintain the original surface brightness.
You need to require annealing in a reducing atmosphere. Hydrogen with at least a -40 DP

My hunch is that some oxide is present on the surface to start with. It may be shiney, but it is there. No vacuum or inert gas can remove existing oxides.

You can try to clean the parts afterward with nitric acid. Be carefull though, you can get rapid attack.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[dgallup]

To answer ome of the questions: The parts are coming in various shades of dull gray, definately not black or straw color. The oxide does not have significant thickness but I'm concerned that any chemical treatment to remove it will affect the tight tolerance dimensions.

I will remove the vacuum option, add the -40 dew point requirement and specify that the parts must be clean and free of oils prior to the anneal.

Thanks for all the responses, you have been a big help.

 

[Quazif]

I have checked with several Heat Treat Facilities in the Chicago area, but I have not suceeded in finding a place that offers H2 atmosphere and 1400 - 1500° F.
Any contacts in Chicago area will be a great help.

 

[EdStainless]

That is the low end of bright annealing. It is doable, but often people are reluctant. Often furnaces will not allow the H2 to turn on to below 1600F. I have seen a box anneal used for this.
You may not be able to get it done.
Vac anneal and chemical clean my be your best bet.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[Quazif]

Thanks Ed,
I just tried out a new source and the samples came back today, they look great. The surface is shiny, bright and silver.
The recepie used was Vaccum annealing (Nitrogen) at 1550 - 1850°F and the parts were ultrasonic cleaned at our facility before sending to Heat Treat.
Seeing different results produced by different sources on the same material, I am led to believe that the condition / Maintanence of the Furnace is also very important.

 

[dgallup]

The parts that Quazif sent me are the most beautiful bright anneal I have ever seen. So it can be done in vacuum. These parts were thermal deburred prior to anneal which should have left some oxide so the anneal definately cleaned up the parts. Now, I'm not sure what to say about the spec other than that it looks like we will have to qualify each individual heat treater.